When there is a desire to use a winch, however, automatically, a critical operation in its use consists in determining the moment when, the cable being almost fully rewound, the winch has to be stopped.
In practice, if the winding is stopped too late, the object linked to its end can strike the platform supporting the winch and thereby even damage this structure and injure the operator working on this platform and even be itself damaged. Conversely, if this winding is stopped too early, the object is situated too far from the platform to be able to be recovered in total safety, without the risk of bodily injury or equipment damage.
Consequently, a visual reference mark is usually placed on the cable which makes it possible to visually determine the moment when the cable can be considered to be sufficiently wound on the winch. The visual reference mark consists, very generally, simply and effectively, of a white colored reference mark, such as an annular marking painted directly on the cable.
Thus, in the case of manual use of the winch, it is the operator situated on the platform who, when rewinding the cable, on seeing the white colored reference mark appear in his or her field of vision, determines the appropriate moment to order the slowing down and the stopping of the winch.
Similarly, in the case of partially or totally automated use, an optical sensor is used, charged with monitoring the travel of the cable and capable of detecting the appearance, within its observation sector, situated in proximity to the platform, of the portion of white colored cable. When the optical device detects the passing of this reference mark, it sends an indication to the means controlling the winch motor informing them that the winch has to be stopped within a short delay.
The use of an optical detection system to detect the appearance of the end-of-winding reference mark offers numerous advantages, among which can be cited the ease of use. In practice, this only requires the positioning, in proximity to the cable, on a mechanical structure linked to the platform, of an optical sensor whose beam is directed toward the cable. However, in the case of an airborne winch system mounted on a platform, installed in the interior of an aircraft for example, and intended to place a given object in water and keep it immersed for a given time, then to rapidly raise the object back onboard the aircraft, the use of an end-of-winding optical detection system, placed at a given distance from the cable and securely attached to the platform, poses at least two difficulties.
The first difficulty consists in producing, without any doubt, the detection by the optical sensor of the white colored reference mark, within a restricted observation zone, while the cable is traveling at a significant speed, possibly reaching 10 m/s for example. In practice, the detection is generally made by means of an optical sensor, configured to emit a narrow light beam and detect the reflection of the light beam by a reflecting object. Consequently, the detection is all the more definite when the detector is closer to the detected object and the object concerned reflects a greater portion of the light beam. Now, as illustrated in FIG. 1, with regard to a cable, with a substantially circular section, only a small portion of the surface of the cable is likely to reflect the light beam in the direction of the sensor. The reflected energy is therefore weak. Consequently, the sensor may detect nothing even in the presence of the portion of cable bearing the white colored reference mark, which is intrinsically more reflecting than the rest of the uncolored metal cable which has a matt appearance.
The second difficulty is linked to the fact that, because of its rapid rise, the cable brings up with it water which is splashed into the space situated between the detector and the cable, in the detection zone. This splashing of water on the one hand causes the efficiency of the detector to be reduced and on the other hand provokes spurious reflections inasmuch as, independently of the presence of the colored reference mark in the detection zone, the water droplets scattered in the detection zone can reflect the beam emitted by the detector with a sufficient intensity to provoke an incorrect detection of the end of the rewinding operation.